1. Field of the Invention
The present invention relates to a direct optical amplifier in a wavelength Division Multiplexing (WDM) optical transmission system.
2. Description of the Related Art
In a WDM optical transmission system, the control of the main signal to a fixed level is a key issue. A first solution that can be considered for controlling the level of the main signal to a fixed level involves monitoring the level of the main signal itself and then controlling this signal to a fixed level (as disclosed in, for example, Japanese Patent Laid-Open Publication No. 103013/01 and Japanese Patent Laid-Open Publication No. 144692/01).
However, the modulating frequency of the main signal is high and the main signal therefore tends to be easily affected by noise. Using a main signal directly for control complicates the achievement of a fixed output control in which the main signal is stable.
However, new methods have been invented (for example, Japanese Patent Laid-Open Publication No. 275530/92 and Japanese Patent Laid-Open Publication No. 257646/01) for maintaining the output of a main signal at a fixed level by controlling an optical signal (a pilot tone signal) to a fixed level, this optical signal having a wavelength that differs from the wavelength of the main signal, and moreover, having a level that is equal to the average level of the main signal, which has a plurality of channels.
The pilot tone signal that is used in these methods has a modulating frequency that is significantly lower than the modulating frequency of the main signal. The pilot signal is therefore relatively impervious to influence from noise and can therefore provide fixed output control of a main signal that is more stable than in a method of directly employing the main signal for control.
Nevertheless, in the pilot tone signal uniform control, control is realized by taking the level of the pilot tone signal as equal to the average level of the main signal and then monitoring only the pilot tone signal. Thus, when the gain of a direct optical amplifier is dependent on wavelength, there is the danger that the pilot tone signal will lose its meaning as the reference light of the main signal. In other words, control is effected on the assumption that the level of the pilot tone signal is equal to the average level of the main signal. However, since the pilot tone signal and the main signal have different wavelengths, the dependency in the gain of the direct optical amplifier on wavelength will result in divergence of the level of the pilot tone signal and the average level of the main signal, which otherwise should be equal, and the pilot tone signal can no longer function as the reference light to control the main signal to a fixed output.
When the pilot tone signal no longer functions as the reference light for fixed output control of the main signal as described above, it should be clear that the main signal cannot be controlled to a fixed level even if the pilot tone signal is blindly controlled to a fixed level. On the contrary, there is the danger that the average level of the main signal, which is supposed to be equal to the level of the pilot tone signal, will go out of control and rise to excessive highs or fall to excessive lows. Excessive increases in the output level result in the occurrence of nonlinearity in the transmission optical fiber, and excessive drops in the output level result in the deterioration of the S/N ratio.
We next refer to FIGS. 1A and 1B, which present schematic views of this problem. FIG. 1A shows a case in which the average level of the main signal rises excessively over the level of the pilot tone signal, and FIG. 1B shows a case in which the average level of the main signal falls excessively from the level of the pilot tone signal. For the sake of simplifying the explanation, the three direct optical amplifiers in both FIGS. 1A and 1B are assumed to have the same gain characteristics.
In FIG. 1A, the net gain of the main signal is greater than the net gain of the pilot tone signal (Since the pilot tone signal is controlled to a fixed level, the gain and loss of the direct optical amplifier in the wavelength of the pilot tone signal are in balance, the net gain is 0, and the output is fixed), and the average level of the main signal, which should have been equal to the pilot tone signal level, becomes progressively greater as a result of relay transmission.
In FIG. 1B, in contrast, the net gain of the main signal is smaller than the net gain of the pilot tone signal, and the average level of the main signal, which should be equal to the level of the pilot tone signal, therefore becomes progressively smaller as a result of relay transmission.
Although the gain can be leveled by means of a multi-relay connection scheme in which a plurality of sets of direct optical amplifiers having different gain characteristics are combined, this scheme could not be considered a real solution to the problem.